Due to ever-increasing energy demands, depletion of fossil fuel, and environmental constraints, the interest in generating green energy at all levels is at an all time peak. Worldwide, governmental incentives and subsidy programs are attracting several customers to install small capacity (ranging from few watts to few kW) renewable energy modules in their premises. Similarly, large companies are building PV solar farms ranging from few hundred kW to few MW or higher capacity. Distributed generation (DG)—power sources connected at one or more locations within the distribution system have brought new issues and problems to the existing power system.
The penetration level of DG systems, such as renewable-energy based DG systems, is growing. As such, the utility companies are facing major challenges of grid-integrating these increasing number sources of power. Challenges such as ensuring voltage regulation, system stability and power quality within standard limits, are at the forefront of these problems.
FACTS devices offer a viable solution to this problem and are being increasingly employed in power systems worldwide. FACTS are defined here as alternating current transmission systems incorporating power-electronic based and other static controllers to enhance controllability and increase power transfer capability. FACTS devices are typically utilized for accomplishing the following objectives:                Voltage control        Increase/control of power transmission capacity in a line, and for preventing loop flows        Improvement of system transient stability limit        Enhancement of system damping        Mitigation of sub-synchronous resonance        Alleviation of voltage instability        Limiting short circuit currents        Improvement of HVDC converter terminal performance        Grid Integration of Wind Power Generation Systems        
Some of the devices/controllers in the family of the FACTS device that have been used for achieving any or all of the above objectives are Static Var Compensators (SVC) and Static Synchronous Compensators (STATCOM), etc.
A static synchronous compensator (STATCOM) is a shunt connected reactive power compensation device that is capable of generating and/or absorbing reactive power whose output can be varied to control specific parameters of an electrical power system. In general terms, a STATCOM is a solid-state switching converter that is capable of independently generating or absorbing controllable real and reactive power at its output terminals when it is fed from an energy source or an energy storage device at its input terminals.
More specifically, the STATCOM is a voltage source converter that produces from a given input of direct current (DC) voltage a set of three-phase AC output voltages. Each output voltage is in phase with and is coupled to the corresponding AC system voltage through a relatively small reactance (which can be provided either by an interface reactor or leakage inductance of a coupling transformer). The DC voltage is provided by an energy storage capacitor.
It is also known in the prior art that a STATCOM provides desired reactive power generation, as well as power absorption, by means of electronic processing of voltage and current waveforms in a voltage source converter (VSC). The STATCOM also provides voltage support by generating or absorbing reactive power at the point of common coupling (PCC) without the need for large external reactors or capacitor banks. Therefore, the STATCOM occupies a much smaller physical footprint.
For purposes of this document, a converter is a general name for both rectifiers and inverters.
It also known that a STATCOM can improve power system performance in areas such as:                Voltage control        Increase/control of power transmission capacity in a line, and for preventing loop flows        Improvement of system transient stability limit        Enhancement of system damping        Mitigation of sub-synchronous resonance        Alleviation of voltage instability        Limiting short circuit currents        Improvement of HVDC converter terminal performance        Grid Integration of Wind Power Generation Systems        Voltage flicker control; and        Control of reactive power and also, if needed, active power in the connected line (this configuration requires a DC energy source).        
The reactive and real power exchange between the STATCOM and the AC system can be controlled independently of one other. Any combination of the real power generation/absorption together with reactive power generation/absorption is achievable, if the STATCOM is equipped with an energy storage device of suitable capacity. With this capability, some extremely effective control strategies for the modulation of the reactive and real output power can be devised to improve the transient and dynamic system stability limits.
The increasing penetration level of DG systems in modern power transmission and distribution systems is presenting several technical challenges. One of these challenges is the voltage variation along the feeder. Traditionally, the direction of electrical power flow has been from the grid towards the loads connected in the distribution feeders. The voltage drop over the feeder length was tackled effectively by adjusting the sending end voltage magnitude or by providing reactive power support at one or more locations in the transmission/distribution feeders. To maintain the voltage at different locations within the standard limits, the utility companies traditionally use a combination of on-line tap changing transformers, and capacitor banks at different locations.
A DG system dominated by wind farms, however, may exhibit an interesting condition, especially at night. At this time, the electrical loads are much lower than their day-time values, given that the wind turbine generator outputs are much higher due to high wind speeds in the night compared to day. This increased power generated from wind farms at night can cause significant amount of power to flow in the reverse direction towards the main grid. Since the present power distribution systems were designed and operated with an important assumption of power always flowing from main grid towards the end users, this reverse power flow condition causes the feeder voltages to rise above their normal rated values. In certain cases, this increase in voltage can exceed the typically allowable limit of ±5%. This is not acceptable to electric utilities.
The problem of reverse power flow presents a major challenge when adding more DG systems to a feeder line. Maintaining the voltage rise within the specified range directly affects the number of DG systems that can be connected on a particular distribution network. When adding additional wind farms to a network, utilities may be forced to install expensive voltage regulating devices in the family of FACTS controllers, such as an SVC or a STATCOM to mitigate this problem.
In light of the above, there is a need for a system, method, and/or device for adapting existing DG systems to support the addition of wind farms and other DG sources without requiring expensive voltage regulation devices.